Polarization and corrosion behavior of 316L stainless steel and alloy AL6X in acid chloride environments

Author

In-Seop Lee

Date of Award

3-1986

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

E. E. Stansbury

Committee Members

C. D. Lundin, C. R. Brooks

Abstract

The pitting susceptibility of 316L stainless steel and alloy AL6X (nominally 20% chromium, 25% nickel, and 6% molybdenum) was evaluated electrochemically and chemically in chloride environments with varing acid pH, ferric ion and chloride ion concentration at room temperature, 135°F, and 170°F. A method was developed for preparing samples to ensure that crevice corrosion does not occur at the metal/epoxy interface prior to general pitting corrosion. The method involved prepassivating samples in 50% HNO₃ + 50% H₂O at 122°F for 30 minutes, painting over the interface between the metal and the epoxy with 1201 Red Enamel and finally removing the prepassivated passive film from the exposed surface by careful hand polishing with 1 micron diamond paste.

This sample preparation allowed confirmation that the pitting potential is decreased with increasing chloride ion concentration and temperature. However higher pitting potentials were observed than previously reported suggesting that crevice corrosion potentials may be erroneously reported as pitting potentials. The critical current density in the anodic polarization curve was not affected significantly with increasing chloride ion concentration. An increase in the anodic maximum current density was observed on decreasing the cathodic potential from which the scan was initiated in determining the polarization curve. This effect was related to oxidation of hydrogen generated in the cathodic potential range and confirmed by related measurements on a platinum electrode.

An effect of heat treatment on the pitting resistance of AL6X was confirmed. Solution annealing in the range of 2050°F-2450°F did not affect the pitting resistance but when solution annealed at 1950°F, Sigma phase formation along the grain boundary was associated with pit initiation. A series of experiments was conducted to further investigate the relationship between electrochemical and chemical methods of determining critical pitting potentials. In the chemical method, the potential was increased by increasing the concentration of ferric ion as ferric chloride while holding the chloride ion concentration constant with sodium chloride. Since the maximum potential that could be produced by ferric ion addition without exceeding the selected chloride ion concentration was 0.76 V (SHE) which was below the electrochemical pitting potential of 0.82 V (SHE), further comparison was made at 135°F based on the decreasing pitting potential with increasing temperature. When 316L stainless steel was immersed in 15g/l FeCl₃6H₂O containing the same chloride ion concentration as 5% NaCl with pH=2 at 135°F, the corrosion potential increased up to 0.47 V (SHE) due to the initial passivation after 2 minutes. This potential is just above the pitting potential of 0.45 V (SHE) determined by electrochemical method in 5% NaCl, pH=2 at 135°F. A decrease in corrosion potential indicated the initiation of pitting corrosion which was confirmed by visual observation.

Recommended Citation

Lee, In-Seop, "Polarization and corrosion behavior of 316L stainless steel and alloy AL6X in acid chloride environments. " Master's Thesis, University of Tennessee, 1986.
https://trace.tennessee.edu/utk_gradthes/13738